As is known, waterglass is understood by a person skilled in the art as meaning glassy, water-soluble alkali metal silicates (i.e. alkali metal salts of silicic acids) solidified from the melt flow, or viscous aqueous solutions thereof. In the waterglass, there are typically 1 to 5 mol of SiO2 per 1 mol of alkali metal oxide (Alk2O—where Alk is the alkali metals lithium, sodium and potassium), and the soda waterglass and potash waterglass are therefore usually also characterized by the quotient of % SiO2 to % alkali metal oxide. In chemical terms, water glasses are therefore alkali metal silicates with a molar ratio (MR) of SiO2 to Alk2O in the range from 0.5 to 8, this molar ratio being understood as meaning the molar ratio of the SiO2 and Alk2O building blocks constituting the alkali metal silicate. They contain oligomeric silicate anions with alkali metal cations as counterions.
In the pure state, waterglasses are colorless glasses which form colloidal clear alkaline-reacting solutions with water at elevated temperature and pressure. Waterglass is usually produced by melting quartz sand together with soda or potash at 1400 to 1500° C., where the silicic acid drives out the CO2. The solidified melt is either traded in the ground state or converted to an aqueous solution with the desired concentration. Aqueous solutions have the advantage of better applicability in a number of different fields of application.
As is known to the person skilled in the art, when producing waterglass in the melting furnace, firstly a melt of alkali metal silicate is formed, which solidifies upon cooling to room temperature to give glass lumps (so-called lump glass). In industry, these glass lumps are dissolved in water in pressurized autoclaves, usually at elevated temperature, for example at ca. 140° C. (so-called dissolution process).
Alternatively, sand can also be dissolved directly by hydrothermal means in concentrated alkali metal hydroxide solution, although only solutions with a relatively low MR can be obtained.
Technical waterglass solutions usually comprise a solids fraction of ca. 30 to 60% by weight.
Waterglass solutions customary in the marketplace contain, irrespective of the method of manufacture, as a consequence of the raw material, small amounts of water-insoluble impurities, these in the case of aqueous solutions customary in the marketplace being in total below 0.1% by weight to 0.01% by weight. These impurities lead to a certain turbidity which is scarcely visible to the eye; according to investigations by the applicant, the specified waterglass solutions customary in the marketplace, when transparency is measured in accordance with DIN EN ISO 7027, have values in the range from 5 to 40 FNU.
All leading manufacturers give warnings in their information with regard to storage and transportation against the risk of frost when dealing with waterglass solutions. In the event of freezing, product separation may in certain circumstances also additionally arise, after which the products cannot be regenerated so as to be usable. In general, it is therefore advised to transport and store the solutions above 5° C. As far as possible, tanks should be positioned in heated works premises or they must, if set up outside, be insulated and equipped with trace heating (PQ internet info “Storing Liquid Silicates, Storage Tank Recommendations”; Crosfield Chemicals “Sodium Silicates” ed. 1976 1-23 Bulk Storage; Diamond Shamrock “Sodium Silicate Handbook” 1982 p. 30). Accordingly, the safety data sheets of all leading waterglass manufacturers warn against the effect of frost; the latter also independently of whether it is a technical product, or whether a very clear filtered solution is delivered for specific applications. (Example from the world's largest manufacturer PQ (Philadelphia Quartz Co.): in the safety data sheets for the products “Sodium Silicate N” (technical-grade) and “N CLEAR” (specifically filtered grade)—molar ratio (MR) of SiO2 to Alk2O for both products 3.3—or “Sodium Silicate V gg STAR” (specifically “brilliant clear” grade)—MR 2.5-0° C. is stated as the lowest storage temperature.)